If I were in your position I’m sure I’d be annoyed too. But this is one of the reasons I use an iPhone instead of an Android phone. With iPhone, you get updates for years after buying your phone, while Android is very hit or miss and company-dependent. This means that I can continue benefiting from improved web features used by Discourse while, unfortunately, you lose support.
It’s not that the software is actively preventing it. It’s that they are nicely informing you that your browser isn’t supported instead of failing silently. As soon as they started using features not supported by your browser, their only options were to let you know or let it fail silently while things don’t work properly. At least until they bring the no-JS version to old browsers that is.
Ending support for a small percentage of users enables them to use new features that benefit the vast majority of users. I know saying that doesn’t help your situation at all, but that’s just the way it is.